1. Field of the Invention
The present invention relates to a process for producing magnetic iron oxide particles for use in magnetic recording and, more particularly, to a process for producing magnetic iron oxide particles suitable for a magnetic recording medium capable of effecting acoustic and image recording in a high density.
2. Description of the Related Art
Recently, there has been an increasing need to a magnetic recording medium having a higher performance such as magnetic tapes, magnetic disks and the like, associated with magnetic recording of sounds and images, miniaturization of reproducing instruments, an enhancement of quality and a great increase of a magnetic information processing capacity. That is, further enhancements of properties such as a high recording density, a high output and the like, have been increasingly demanded, which requires in turn magnetic particles used in the magnetic recording mediums to be much fine particles and to have a higher coercive force and a higher saturation magnetization characteristic.
Although making magnetic particles finer is one of the most effective methods for reducing noises, it causes inevitably a reduction of the saturation magnetization of the particles as well as reductions of a filling ratio and an orientability of the magnetic particles in a magnetic layer containing them dispersed as the sizes of the particles are reduced. Therefore, there is a great need to further enhance the magnetic properties of the magnetic particles such as saturation magnetization and remanent magnetization so that a higher recording density and a higher output can be imparted to the magnetic recording mediums. Various techniques have been proposed to improve the magnetic properties of the magnetic particles for use in the magnetic recording mediums. For example, there have been known a method for improving print-through as well as the saturation magnetization by heat-treating iron oxides to density the crystalline texture (for example, Japanese Patent KOKAI (Laid-Open) No. 58-199725), or a method for improving print-through as well as the saturation magnetization by modifying maghemite particles with a combination of ferrous compounds and zinc compounds and, if necessary, by further heat-treating the coated particles to avoid deterioration of the print-through and the saturation magnetization due to the coating-treatment when maghemite particles are modified with cobalt compounds for improvement of coercive force (for example, Japanese Patent KOKAI (Laid-Open) Nos. 53-87961, 60-208805, and 61-4202). In the former case, however, an attempt to obtain sufficient print-through tends to form .alpha.-Fe.sub.2 O.sub.3, to cause unavoidably reduction of the saturation magnetization, and in the latter case, the coercive force variers to a large extent with time though a certain degree of improvement of the coercive force and the saturation magnetization. Therefore, there have not been a few of difficulties to be improved yet.